Mixed Layer Modeling of Intraseasonal Variability in the Tropical Western Pacific and Indian Oceans

Abstract

Sea surface temperature (SST) variations associated with the atmospheric intraseasonal oscillation in the tropical Indian and western Pacific Oceans, are examined using a one-dimensional mixed layer model. Surface fluxes associated with 10 well-defined intraseasonal events from the period 1986?93 are used to force the model. Surface winds from the European Centre for Medium-Range Weather Forecasts daily analyses and SST from the mixed layer model are used to compute latent and sensible heat fluxes and wind stress with the TOGA COARE bulk flux algorithm. Surface freshwater flux is estimated from the Microwave Sounding Unit precipitation data. Net shortwave radiation is estimated, via regression analysis, from outgoing longwave radiation. An idealized diurnal cycle of shortwave radiation is also imposed. The intraseasonal SST variation from the model, when forced by the surface fluxes estimated from gridded analyses, agrees well with the SST observed at a mooring during the COARE. The model was then integrated for the 10 well-defined intraseasonal events at grid points from 75° to 175°E at 5°S, which spans the warm pool of the equatorial Indian and western Pacific Oceans. The one-dimensional model is able to simulate the amplitude of the observed intraseasonal SST variation throughout this domain. Variations of shortwave radiation and latent heat flux are equally important for driving the SST variations in the western Pacific, while latent heat flux variations are less important in the Indian Ocean. The phasing of the intraseasonal variation of precipitation relative to wind stress results in little impact of the freshwater flux variation on the intraseasonally varying mixed layer. The diurnal cycle of shortwave radiation is found to significantly increase the intraseasonal amplitude of SST over that produced by daily mean insolation.